Spider bites are usually harmless, and spiders don't usually bite unless threatened.

Spider bites can cause redness, pain and swelling, or you might not notice them at all. Many other bug bites and skin sores cause redness, pain and swelling. So unless you actually saw a spider bite you, it's difficult to be certain that your wound was caused by a spider.

Worldwide only a few species of spiders have fangs long enough to penetrate human skin and venom strong enough to hurt humans. Among these are widow spiders, with about 30 species, and recluse spiders, with more than 140 species worldwide.

Typically, a spider bite looks like any other bug bite — a red, inflamed, sometimes itchy or painful bump on your skin — and may even go unnoticed. Harmless spider bites usually don't produce any other symptoms.

Many skin sores look the same but have other causes, such as a bacterial infection.

Bites from some spiders, such as widow spiders and recluse spiders, might cause serious signs and symptoms.

Widow spider bites

Signs and symptoms of a widow spider bite can include:

• Redness, pain and swelling. You might have pain and swelling around the bite, which can spread into your abdomen, back or chest.
• Cramping. You might have severe abdominal rigidity or cramping, which is sometimes mistaken for appendicitis or a ruptured appendix.
• Nausea, vomiting, tremors or sweating. You might experience nausea, vomiting, tremors or sweating alone or in combination.

Symptoms can last 1 to 3 days.

Recluse spider bites

Signs and symptoms of a recluse spider bite can include:

• Increasing pain over the first eight hours after the bite
• Fever, chills and body aches
• A bite wound with a pale center that turns dark blue or purple with a red ring around it
• A bite wound that grows into an open sore (ulcer) with the skin around it dying

When to see a doctor

Seek medical care immediately if:

• You were bitten by a dangerous spider, such as a widow or recluse.
• You're unsure whether the bite was from a dangerous spider.
• You have severe pain, abdominal cramping or a growing wound at the bite site.
• You're having problems breathing or swallowing.
• The area of the sore has spreading redness or red streaks.

Severe spider bite symptoms occur as a result of the venom that the spider injects. Symptom severity depends on the type of spider, the amount of venom injected and how sensitive your body is to the venom.

Risk factors for spider bites include living in areas where spiders live and disturbing their natural habitat. Widow spiders and recluse spiders like warm climates and dark, dry places.

Widow spider habitat

Widow spiders can be found throughout the United States, except Alaska, and are more common in the rural South. They're also found in Europe. They are more active in the warmer months and prefer to live in:

• Sheds
• Garages
• Unused pots and gardening equipment
• Woodpiles
• Closets and cupboards during cold weather

Recluse spider habitat

Recluse spiders are found most commonly in the southern half of the United States and in South America, where they are known as brown spiders. These spiders are so named because they like to hide away in undisturbed areas. They are most active in the warmer months. Indoors, they prefer to live:

• In cluttered basements and attics
• Behind bookshelves and dressers
• In rarely used cupboards

Sometimes they get mixed up in bed linens and clothing, causing many bites to occur in the early morning.

Outside, they seek out dry, dark, quiet spots, such as under rocks or in tree stumps.

Rarely, a bite from a widow spider or recluse spider is deadly, particularly in small children.

A severe wound from a recluse spider can take weeks or months to heal and leaves large scars.

Spiders usually bite only in defense, when being trapped between your skin and another object.

To prevent spider bites:

• Learn what dangerous spiders look like and their preferred habitats.
• Wear a long-sleeved shirt, hat, long pants tucked into socks, gloves and boots when handling stored boxes or firewood and when cleaning out sheds, garages, basements, attics and crawl spaces.
• Inspect and shake out gardening gloves, boots and clothing before use.
• Use insect repellents, such as DEET. Carefully follow directions on the package.
• Keep insects and spiders out of the house by installing tightfitting screens on windows and doors, sealing cracks where spiders can come in, and using safe indoor insecticides.
• Reduce debris or remove piles of rocks or lumber from the area around your home and avoid storing firewood against the walls of your home.
• Make sure beds aren't pushed against the wall and that only the legs of the bed touch the floor. Don't store items under the bed and don't let bedding drag on the floor.
• Remove spiders and spiderwebs from your home.
• If a spider is on your skin, flick it off with your finger rather than crushing it against your skin.
• When cleaning tarantula enclosures, wear gloves, a surgical mask and eye protection.

Spider bites can be mistaken for other skin sores that are red, painful or swollen. Many skin sores attributed to spider bites turn out to have been caused by bites from other bugs, such as ants, fleas, mites, mosquitoes and biting flies. Skin infections and other skin conditions, even burns, can be mistaken for spider bites.

Your doctor will likely diagnose a spider bite based on your history and your signs and symptoms. The process might involve determining whether anyone saw a spider bite you, having an expert identify the spider, and ruling out other possible causes of the signs and symptoms.

Black widow identification

Some clues for identifying black widow spiders include:

• Shiny black body with long legs
• Red hourglass shape on the belly
• Length of entire body, including legs, about 1 inch (2.5 cm) across

Brown recluse identification

Some clues for identifying brown recluse spiders include:

• Golden or dark brown body with long legs
• Dark violin shape on top of the leg attachment segment
• Six eyes — a pair in front and a pair on both sides — rather than the usual spider pattern of eight eyes in two rows of four
• Central body is about 1/2 inch (1.2 cm) across

Most spider bites usually heal on their own in about a week. A bite from a recluse spider takes longer to heal and sometimes leaves a scar.

First-aid treatment for spider bites includes the following steps:

• Clean the wound with mild soap and water. Apply an antibiotic ointment three times a day to help prevent infection.
• Apply a cool compress over the bite for 15 minutes each hour. Use a clean cloth dampened with water or filled with ice. This helps reduce pain and swelling.
• If possible, elevate the affected area.
• Take an over-the-counter pain reliever as needed.
• If the affected area is itchy, an antihistamine, such as diphenhydramine (Benadryl) or certirizine (Zyrtec), might help.
• Observe the bite for signs of worsening or infection. You might need antibiotics if the bite develops into an open wound or becomes infected.

For pain and muscle spasms, your doctor might prescribe pain medicine, muscle relaxants or both. You might also need a tetanus shot.

Black widow antivenom

If a black widow bite is causing severe pain or life-threatening symptoms, your doctor might recommend antivenom, which is usually given through a vein (intravenously). Symptoms usually ease within about 30 minutes of receiving the antivenom. Antivenom can cause serious allergic reactions, so it must be used with caution.

If you've been bitten by a spider that you suspect is dangerous, call your primary care doctor or go to an urgent care center. If your doctor has online services, an option may be to email a photo of the spider to your doctor.

What you can do

To help your doctor understand your symptoms and how they might relate to a spider bite:

• Bring the spider or a photo of the spider with you, if you can do so safely
• List any symptoms you're experiencing
• List questions to ask your doctor

Some basic questions you might want to ask include:

• Is this a dangerous spider bite?
• If this isn't a spider bite, what are possible causes for my symptoms?
• Do I need any tests?
• How long will my symptoms last?
• What is the best course of action?

What to expect from your doctor

Your doctor is likely to ask you a number of questions. Being ready to answer them might reserve time to go over any points you want to spend more time on. Your doctor might ask:

• When did you begin experiencing symptoms?
• What were you doing in the hours before your symptoms started?
• Have your symptoms gotten worse?
• Does anything relieve your symptoms or make them worse?